Make .bak (backup) files have a regular file icon

I think the right icon for these types of files is the icon for
the filename without the ".bak" suffix.

But, this should be a quick improvement to the current implementation

```
 session_migration-ubuntu
 session_migration-ubuntu-wayland
 user-places.xbel
󰁯 user-places.xbel.bak
 user-places.xbel.tbcache
```

src/theme/icon.rs

@@ -341,7 +341,7 @@ impl IconTheme {
             ("avi", "\u{f008}"),             // ""
             ("avro", "\u{e60b}"),            // ""
             ("awk", "\u{f489}"),             // ""
-            ("bak", "\u{f006f}"),            // "󰁯"
+            ("bak", "\u{f016}"),             // ""
             ("bash_history", "\u{f489}"),    // ""
             ("bash_profile", "\u{f489}"),    // ""
             ("bashrc", "\u{f489}"),          // ""